The need to eliminate lead-based octane enhancers in gasoline has provided an incentive for the development of processes to produce high octane gasolines blended with lower aliphatic octane boosters. Supplementary fuels are being examined by the petroleum refining industry. Lower molecular weight alcohols and ethers, such as isopropyl alcohol (IPA) and diisopropyl ether (DIPE), are in the boiling range of gasoline fuels and are known to have a high blending octane number. They are also useful as octane enhancers. In addition, by-product propylene from which IPA and DIPE can be made is usually available in a fuels refinery, typically as a C.sub.3 aliphatic stream which is rich in both propylene and propane.
The preparation of DIPE from propylene chemically proceeds by two sequential reactions where propylene is first hydrated to IPA (1) followed by reaction of the alcohol with the olefin (2) or bimolecular reaction of the alcohol (3) (Williamson synthesis) according to the equations, EQU CH.sub.3 CH=CH.sub.2 +HOH.rarw..fwdarw.CH.sub.3 CHOHCH.sub.3( 1) ##STR1##
When DIPE is produced via reaction (3), twice as much IPA is required than when DIPE is produced via reaction (2). Since hydration reactions, for example reaction (1), are generally more difficult to perform than etherification reactions, the production rate of the alcohol limits the overall sequence and it is desirable to limit the formation of DIPE from reaction (3) and increase the formation of DIPE from reaction (2). Side reactions that can occur in this process are the reaction of propylene with itself to make C.sub.6 olefins and the reaction of C.sub.6 olefins with propylene to make C.sub.9 olefins. These reactions are considered undesirable since they result in low value polygasoline with low octane and no oxygen content.
The synthetic production of IPA and DIPE is well known. Among the earliest processes for the production of IPA and DIPE were the so-called indirect hydration processes. In the indirect hydration process, a selected olefin feed is absorbed in a concentrated sulfuric acid stream to form an extract containing the corresponding alkyl ester of the sulfuric acid. Thereafter, water is admixed with the ester-containing extract to hydrolyze the ester and to form the desired alcohol and ether which are then recovered, generally by stripping with steam or some other heating fluid. A diluted sulfuric acid stream is thereby produced. This acid stream is then generally treated to concentrate the sulfuric acid stream for recycle to the absorption stage.
In the indirect hydration process, the use of sulfuric acid as a catalyst presents certain problems. First, severe corrosion of process equipment can occur. Second, separating the produced ether from the sulfuric acid can be difficult. Third, a substantial quantity of waste sulfuric acid is produced in the concentration of the catalyst for recovery. Because of these problems, it has been found that the process of synthesizing DIPE by using concentrated sulfuric acid is not commercially viable. Clearly, there was a need for a more direct manner of bringing about the hydration reaction.
This need was addressed by so-called direct hydration processes using solid catalysts. In the direct hydration process, an olefinic hydrocarbon such as propylene is reacted directly with water over a solid hydration catalyst to produce an intermediate IPA stream from which the product DIPE can be formed. Development work using direct hydration focuses on the use of solid catalysts such as active charcoal, clays, resins and zeolites. Examples of olefin hydration processes which employ zeolite catalysts as the hydration catalyst can be found in U.S. Pat. Nos. 4,214,107, 4,499,313, 4,857,664 and 4,906,187.
The use of zeolites as hydration catalysts has the disadvantage of being expensive in comparison to other catalysts, for example, ion exchange resin catalysts. Also, in comparison to ion exchange resin catalysts zeolites do not operate as well at the relatively low temperatures required for hydration and etherification. Furthermore, zeolites have a strong tendency to form DIPE from reaction (3) instead of reaction (2). They also have a strong tendency to produce substantial amounts of undesirable polygasoline from the reaction of propylene with itself.
In many of the direct hydration processes developed by the refining industry, the hydration reactor is operated to produce both IPA and DIPE. In other words, the hydration of olefins to IPA and the etherification of IPA to DIPE occurs in a single reactor. After separation, the IPA is then recycled to the hydration reactor to produce additional DIPE. For example, in U.S. Pat. No. 5,102,428 (issued to Owen et al.) a C.sub.3 hydrocarbon feed stream comprising propylene and propane is fed along with water to a single hydration/etherification reactor containing an acidic catalyst. Both hydration and etherification reactions occur in this single reactor. The reactor effluent containing DIPE, IPA and propylene is then passed to a high pressure separator/extractor wherein the effluent is contacted by water to remove a substantial amount of the IPA. The resulting stream is flashed to remove propylene and fractionated to remove the remaining IPA. The aqueous IPA stream and the IPA recovered from fractionation are recycled to the hydration/etherification reactor for reuse. Unconverted propylene is polymerized to form polygasoline using a metallosilicate catalyst rather than recycled to the hydration/etherification reactor to produce additional IPA. This is highly undesirable since it limits the oxygenate production and produces substantial amounts of polygasoline which has low octane.
Using a single reactor for hydration and etherification can present a problem because the conversion of propylene and water to IPA takes place at a higher temperature than the conversion of IPA to DIPE. By having both reactions take place in a single reactor, the conversion of IPA to DIPE must occur at a higher temperature than desired and the conversion of IPA to DIPE is decreased.
Some direct hydration processes use two reactors so that any unreacted IPA formed in the first reactor can be sent to a second reactor to further etherify the IPA. For example, in U.S. Pat. No. 4,935,552, propylene, water and IPA are introduced to a first reactor containing a hydration/etherification catalyst to form an effluent which consists of IPA, DIPE and unreacted reactants. The effluent is then flashed to remove propylene and extracted with water to transfer the IPA to the aqueous phase. A portion of the resulting hydrocarbon phase containing high purity DIPE is recycled to the first reactor in order to control the temperature. This is undesirable since recycling a product in an equilibrium-limited reaction will reduce conversion of the reactants, thereby limiting production of IPA and DIPE. The aqueous IPA phase is passed to a second reactor (catalytic distillation unit) where additional etherification of the IPA to form DIPE takes place. In this second reactor, DIPE is made exclusively by reaction (3) since the propylene has all been removed. This means the first reactor has to make twice as much IPA to make the same amount of DIPE.
U.S. Pat. No. 5,144,086 (issued to Harandi et al.) discloses an integrated IPA and DIPE process that has two separate reactors, one for the exclusive formation of IPA and the other dedicated to etherification. In this process, propylene and water are introduced to a first hydration reactor containing an acidic hydration catalyst to produce an IPA-containing effluent that is substantially free of DIPE. After separating the effluent stream to remove the propylene and propane, the IPA-containing stream is passed to an etherification reactor containing a zeolite catalyst to produce an etherification effluent that contains DIPE, water and propylene. This process produces DIPE by reaction (3) rather than reaction (2) and therefore requires twice as much IPA to be made. The effluent from the etherification reactor is then fractionated to produce the DIPE product. This process suggests using one separation zone (distillation column) to remove C.sub.3 hydrocarbons and water from the intermediate IPA stream and another separation zone to remove C.sub.3 hydrocarbons and water from the DIPE product stream, resulting in a considerable addition to the capital expense of the integrated process.
There is a need for an integrated IPA and DIPE process which utilizes common separation zones to remove C.sub.3 hydrocarbons and water from the IPA and DIPE streams, and which maximizes conversion in each stage, minimizes formation of DIPE from dehydration (reaction 3), and minimizes polygasoline formation. And it is the object of this invention to fill this need.